My present invention relates to a method of and an apparatus for the production of hollow articles of thermoplastic synthetic resin by a suction-blowing process is which a tubular preform of the thermoplastic synthetic resin material is received between two mold halves which can be moved relatively into open and closed positions, can be locked in a closed position so that the preform is located within the mold cavity defined between the mold half, at least the lower side of the mold cavity can be closed by a slider, suction can be applied at least at the lower portion of the mold and the preform, subject to the suction and enclosed in the mold is expanded by the blowing or by reason of the expansion of the previously preblown preform under the pressure reduction brought about by the suction in the mold.
The suction-blowing process used heretofore for the production of hollow articles from thermoplastic synthetic resin generally has provided the shaping tool, i.e. the mold or die, directly adjacent the extrusion head or coextrusion head from which the tubular preform of thermoplastic synthetic resin emerges, usually downwardly. The tubular extrusion can be an extrusion of one material or can be formed by coextrusion, i.e. with one thermoplastic material laminated over another in the extrusion process.
The hanging tubular preform in the extrusion or coextrusion head can be drawn into or through the closed blow-molding tool by suction until the tubular preform, with the aid of the suction cushion, arrives at its final position in the mold. The mold is then closed at the side of the extrusion head and blowing can be effected.
A suction-blowing machine has a number of advantages over a conventional blow-molding machine as will be apparent, for example, from German patent document DE 37 22 340 A1. Since the tubular preform is drawn by suction into a closed blow mold, is undesirable squeezing of the tubular preform cannot occur. Such squeezing is common when the blow mold is closed on a preform in the conventional blow-molding operation without the suction facility described above. Since the suction-blown article also does not have a weld seam, the mold-closing force need only be sufficient to hold the mold closed against the blowing pressure which may result, as noted, by application of suction externally of the hollow body. The closing operation, therefore, requires significantly lower pressure than for closing a mold onto a synthetic resin tube. Furthermore, expensive mold locks with position and speed controllers can be eliminated. Such devices are essential for conventional blow-molding machines in which the mold halves close upon the synthetic resin material and form seams in the finished body.
A further drawback of earlier systems resulting from the fact that the mold operates discontinuously, is that the extrusion head must be capable of storing the thermoplastic synthetic resin which is generally fed continuously by one or more screw or worm extruders. The expensive storage head must be capable of ejecting the tubular preform rapidly, generally over a period of several seconds, as may be required for the suction-blowing operation so that excessive storage of the thermoplastic is avoided. The storage head therefore must have sufficient storage capacity for the thermoplastic synthetic resin which would be extruded until the mold reopens and the finished article is removed. The storage space requires a piston for discharging the stored thermoplastic.
Such systems as described in German patent documents DE 36 20 144 A1 and DE 36 35 334 A1 create a variety of operational preforms and, of course, are expensive and require significant maintenance.
It is, therefore, the principal object of the present invention to provide an improved method of and apparatus for the production of hollow articles from thermoplastic synthetic resin material which avoids the drawbacks of the earlier system and can operate more efficiently in a suction-blowing mode than has hitherto been the case.
More specifically, it is an object of the invention to provide a method of suction-blowing hollow articles from thermoplastic synthetic resin materials whereby the procedure is simplified, the articles are made with greater precision, and problems hitherto encountered and resulting from the need to store thermoplastic synthetic resin at the extrusion head are avoided.
It is also an object of the invention to provide an apparatus for producing hollow articles by the suction-blowing process which enables both extrusion and shaping of the tubular preform to be carried out more simply and with better control than has hitherto been the case.
These objects and others which will become apparent hereinafter are attained, in accordance with the invention by decoupling the material-shaping part of the method from the extrusion part of the method and thereby enabling a tubular preform which has previously been extruded to be shaped while a tubular preform for a subsequent shaping continues to be extruded by the extrusion head.
In other words with the invention the extrusion or coextrusion resulting in formation of the tubular preform is decoupled from the process of sucking the tubular preform into the cavity and expanding it, i.e. the article-shaping process, so that during the formation of a finished article, a next preform continues to be extruded and continuously or discontinuously can be ejected from the head and is prepared for introduction into the mold cavity. Because of the decoupling of the procedure by which a tubular preform is drawn by suction into the mold and then shaped by suction blowing and the extrusion, numerous advantages can be seen. For example, the suction blowing eliminates the need for a storage head and enables the use of a six-axis robot to be used in combination with the transfer of the preform to a 3D blow mold since the arms of the robot need only engage the preform between the extrusion head and the mold to enable transfer of the preform between the head and the mold. The invention allows sequential coextrusion with two materials of different stiffness and hence with different expulsion rates from the coextrusion head, since the extrusion rate need not be matched to the shaping operation or vice versa. The matching of cycling times of the extrusion head and the shaping unit is no longer necessary since the six-axis robot can pick up the tubular preform at any point for the transfer.
The method thus comprises the steps of:
(a) extruding a tubular blow-moldable thermoplastic synthetic resin preform with an extrusion head supplied with the thermoplastic synthetic resin by at least one extruder;
(b) transferring the tubular preform to a mold cavity formed by closing two mold halves and displacing a slider across the mold cavity at least at a bottom of the mold cavity;
(c) drawing a suction in the mold cavity at a lower portion thereof;
(d) blowing the tubular preform in the cavity to shape an article therefrom; and
(e) during steps (b) through (d) extruding another tubular preform for subsequent shaping into an article whereby the shaping of preforms into articles is decoupled from the extrusion of preforms and simultaneously with the shaping of the preforms, tubular preforms are continuously or discontinuously extruded by the head.
An apparatus according to the invention can comprise:
an extrusion head connected to at least one extruder for extruding a tubular blow-moldable thermoplastic synthetic resin preform;
a mold spaced from the head and having a mold cavity adapted to receive the tubular preform, a slider being displaceable across the mold cavity at least at a bottom of the mold cavity;
means connected with the mold for drawing a suction in the mold cavity at a lower portion thereof; and
a preform handler positioned to grip a preform extruded by the head for transferring the preform to the cavity for shaping into an article whereby the shaping of preforms into articles is decoupled from the extrusion of preforms and simultaneously with the shaping of the preforms, tubular preforms are continuously or discontinuously extruded at the head.
With this apparatus, a preform handler which can be a six-axis robot, is provided for removing the tubular preform from the extrusion or coextrusion head and carrying it to the article cavity. The use of the preform holder, which can be a fully programmable six-axis robot, the suction-blowing machine is associated with the extruder/coextruder but has its shaping part decoupled therefrom so that the extrusion or coextrusion head can continue to prepare a further tubular preform during the period in which the handler carries a previously produced preform to the shaping components and returns to the head.
According to a feature of the invention, the handler is raisable and lowerable and thus its arms can be brought into the region of the nozzle of the head or displaced toward the mold which lies below the head. Once the tubular preform is picked up by the arms, extrusion can continue. The handler advantageously can be equipped with means for cutting, welding and/or preblowing the tubular preform, thereby eliminating the need for a separate cutting device for separating the desired length of extruded material as the respective preform from the continuously extruded tube. The preblowing on the handler, e.g. with the aid of a blowing piercer which is integrated into the handle, facilitates the insertion of the tubular preform into the cavity of the closed blow mold and thus permits a calibration of the article.
It has been found to be advantageous to provide the preform handler with two gripper arms movable toward and away from one another and which are configured with complementary half-shell-shaped cavities in which vacuum nozzles open. Following the removal of a preform from the head and closure of the gripper arms so that the preform is squeezed closed at the head and is held by the arms, the application of suction of these nozzles causes an expansion of the end of the tubular preform held in the last-mentioned cavity so that the preform walls are drawn against the cavity walls. The expanded end of the tubular preform is thereby stabilized and the piercing tool can thus be pressed into the stabilized end to enable preblowing of the preform.